The invention relates to a temperature switch having a rod-like temperature sensor.
DE-B-25 00 586 discloses a temperature switch whose sensor comprises a metal tube, which serves as the expansion element and which transfers its expansion via an inner rod made from a material with a lower thermal expansion coefficient to the switch. This sensor is provided with a quartz tube engaged over it and which ensures that the sensor does not constitute an electrical bridge of the air gap between the heating resistors and the heated glass ceramic plate. As the quartz tube is radiotransparent it scarcely prevents the transmutation of radiation heat.
DE-U-78 26 549 discloses a regulator for gas cookers, in which a further metal tube is engaged over the sensor tube and is spaced therefrom. This tube is intended to prevent the gas flame from directly striking the sensor.
EP-B-116 861 discloses a temperature switch, which is located on a web of insulating material of the radiant heater in which it is fitted and which partly protects it against radiation, so that a temporary response delay is brought about. This makes it possible to bring the radiant heater to a higher temperature level during the preliminary heating or cooking phase and which is then lowered to a steady state during further operation, which ensures that there is no damage to the glass ceramic plate in continuous operation. Much the same can be gathered from EP-B-150 087, in which the outer tube belonging to the temperature sensor is constructed as a quartz glass tube. The sensor tube is provided with a coating of an infrared reflecting material to ensure that the infrared radiation allowed to pass through the tubular element does not reach the inner rod.
From the earlier dated, but not previously published German patent application P 38 21 496 a temperature sensor is known, whose outer sensor tube, which forms the expansion standard, comprises a material mainly absorbing the radiation from the radiation source and e.g. in the form of a completely sintered ceramic material, preferably cordierite.
Temperature switches with such rod-like differential expansion members as sensors, in which the outer tube belonging to the sensor is made from fused silica can admittedly be made relatively thin and as a result of the good insulating characteristics of the fused silica solve the problem of the breakdown strength between the heating resistors and the hotplate, but are relatively susceptible to breakage. A break or even minor damage at one of the ends has an immediate effect on the switching accuracy and can therefore be prejudicial to the complete means to be monitored. It is difficult to apply with the necessary resistance coatings to the fused silica and can produce within the sensor a type of "greenhouse effect", which has an favorable influence on the control behavior.
It would therefore be desirable to use the regulator construction known from DE-B-25 00 586 with an inner reference standard rod and an outer expansion tube, e.g. of metal. However, with the overengaged fused silica tube according to said specification, the switching amplitudes of the regulator are too limited, so that it switches too often and exceeds the admissible "click rate" prescribed as a result of radio and mains interference. In addition, during the first heating switching off takes place earlier than desired. Thus, during the first heating it is possible to exceed the maximum continuous temperature necessary for protecting the glass ceramic without putting it at risk, so that shorter preliminary heating times can be obtained. Moreover the fused silica tube is relatively expensive and susceptible to breakage.